1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method of retransmitting data through a reverse link in a packet data system using ARQ(automatic repeat request)
2. Discussion of the Related Art
Generally, 1× DO(data only) is a mobile communication technique enhancing data transmission rate over 2 Mbps from cdma2000-1× called the 2.5 generation mobile communication. On the contrary, 1×-EV DV is a service system realizing a data transmission rate of 2˜3 Mbps in a mobile section as well as supporting a voice service.
Namely, 1×-EV DV is a general term for the standardization enabling to support a high-speed packet data service as well as the previous voice service based on 1× technology of synchronous cdma2000 RTT.
Compared to the previous 1× RTT technology, 1×-EV DV accepts an adaptive modulation & coding(hereinafter abbreviated AMC) technique and an HARQ system in a forward link . Yet, for a reverse link, channel addition to support the AMC technology and HARQ system is just made thereto.
Generally, link adaptation includes power control and rate control.
In the rate control, a receiving end adjusts a rate of a transmission terminal through power variation of a received signal, for which a power level of the signal received by the receiving end should fluctuate.
Yet, an objective of the power control is to control to lead a level of power, which is received by the receiving end, into a level in which modulation and coding techniques used by the current radio operate at a demanded level. Hence, it is difficult to use both of the power control and the AMC technique simultaneously.
The power control settles a near-far problem occurring at a reverse link. An objective of the power control is to adjust power levels of all terminals received by a base transceiver system into a predetermined level by differing transmission powers of the terminal far from the base transceiver system to the terminal near to the base transceiver system.
Compared to the forward link, the reverse link has the near-far problem in general so as to need the power control absolutely. Therefore, it is difficult to apply the same AMC technique of the forward link thereto.
Meanwhile, the HARQ system combines HARQ(automatic repeat request) through error detection with the previous forward error restoration coding system.
Generally, there are three kinds of the HARQ systems, Type-I of the HARQ system, then an error occurs at the first transmission, retransmits the same information so that a receiving side uses a chase combining form.
Type-II and type-III of the HARQ systems increase redundancy in the respective transmissions. A receiving side combines code of the first transmission signal with of that of retransmission signal so as to reduce a code rate. Namely, compared to the type-I of the HARQ system, type-II and type-III of the HARQ systems attain coding gains, respectively.
In this case, a classification between type-II and type-III of the HARQ systems is made in a following manner. The type-II is taken if each of the transmission information fails to be self-decodable. Andy the type-III is taken if each of the transmission information is self-decodable.
There exists dissimilarity between the forward and reverse link channels. Such dissimilarity makes it difficult to apply the technologies of increasing data throughput in the previous forward link to the reverse.
Generally, the HARQ system considered by the current reverse link takes care of the following items.
First, if a turbo code rate is ¼, a receiving end uses the type-I of HARQ and applies chase combining thereto.
Second, if the turbo code rate is ½. the receiving end uses both of the type-II and the type-III of HARQ so as to use the incremental redundancy.
The use of such systems is supported by the fact that a minimum code rate of the turbo code as an encoding system is ⅕.
The first system, when the turbo code of ¼ code rate is used, having already attained a sufficient coding gain, has no big difference of gain in using the incremental redundancy.
And, the second system enables to attain a big coding gain using the incremental redundancy when the code rate is ½.
There occur some problems in using the above-mentioned systems.
For reference, a redundancy code is a code that a redundant code series are added to the code series required for expressing original information in order to detect or amend errors occurring in the process of transmitting data, which is called “redundancy” in brief. When a packet having NACK is retransmitted it is called “incremental redundancy” that another redundancy having failed to be transmitted in the previous packet is transmitted.
First of all, as explained in the foregoing description, the power control is carried out so as to keep a predetermined level of quality by adjusting a level of receiving power in the reverse link.
Yet, when the chase combining of the first system is used, the retransmission consumes excessive energy amounting to that of the first transmission in result.
Namely, even though the packet error in the reverse, link can be settled mostly by adding little energy to the energy. of the first transmission, a signal having the same energy of the first transmission signal is retransmitted so as to waste energy.
Moreover, when the receiving end executes the chase combining, it is unable to apply the dedicate data rate control for the reverse link traffic, which is currently being considered, to the packet to be retransmitted.
Therefore, a new link adaptation method is required in order to settle the problem of the first system as well as increase the advantageous coding gain of the second system.